you posted this pic a while back.... does it have a red feather duster looking thing that comes out? I have these in my tank, but cant get a good shot of them.
Mini Koko Worms
Koko worms create calcium based shells like that. The bigger ones that are regular feather duster size are much prettier than a typical feather duster. But DANG expensive i think because they usually form along lps skeletons or live rock and removing one is all but impossible most times. Check out the corals(particularly lps) in a lfs you'll notice an empty shell on some of them at some point. I have 2 smalls ones on my torch.
Open full size he's that big? or the tube opening? If open full size he's that big they get to be quite a bit larger. A centimeter or 2 in diameter. You may see him grow some. They come in all sizes and colors though and there is no way of really telling.